The present invention relates generally to press vibration monitoring and, more particularly, to a method of generating a press load/speed vibration severity capacity zone chart for the determination of press/die long-term operating reliability during production operation and to an apparatus utilizing the information generated by the above method in monitoring press vibration severity.
The traditional, state of the art method for calculating the tonnage of the die is mainly by a means of static load calculation. A given die has a certain material shear length and a stock material with a certain thickness. From this, the tonnage of the die or the force necessary to shear or form the part may be calculated. Traditional press sizing has been based on "static" die shear loading as calculated using the equation;
[Shear Length (in.)][Thickness (in.)][S.sub.s (lb/in.sup.2)]=Shear Load (lb).
This load (plus forming and blanking static loads) have traditionally been considered the only significant load and thus the peak dynamic load of the press. Generally, on shorter stroke machines at speeds below 300 strokes per minute, dynamic effects are not a major influence on die application severity. As press speeds are increased, however, there are several other dynamic influences which become present, thereby creating additional press loadings in addition to increases produced by the actual shear loading above the traditional static calculated value. In many cases, these dynamic loads surpass the shear load as the peak dynamic load. In addition to greater effective shear loads, additional impact forces are created as press speed increases, which further contributes to the vibration of the press structure.
It has been found through experimentation that as the press speed increases there are many additional loads that occur that are not present at slower press speeds. There are actually several different sources of additional die load parameters that many press owners do not know exist. At higher speeds, even though not exceeding the capacity of the press, the press requires more force to make the part, which in turn creates a different set of more severe vibration conditions.
At higher press speed, in the press structure, the loads are applied much more quickly, are released more quickly, and in general are producing a much stronger shock wave which is sent through the press structure. By increasing the speed of the press, the velocity at any given point above bottom dead center is increased, thereby increasing the impact forces of the punches on the stock material. These impact forces are related to the square of the velocity. Therefore, press speed is one of several factors increasing vibration in the press. By running the press at higher speeds more severe vibration is transmitted through the press.
A second factor contributing to press vibration is the stroke length, which increases the impact forces and loading on the press. A third factor is the contact distance of the die punches and stripper plate above bottom dead center. The higher these components contact above bottom dead center, the greater the impact velocity and, therefore, the more severe the vibration level.
Additionally, as the strain rate of the stock material is increased, which happens normally when press speed increases, the shearing properties of the stock material change. The effective shear strength of the stock material increases significantly and actually starts to approach the ultimate strength of the stock material. As the dynamic loads are developed more quickly and/or as the speed of the machine increases, the yield strength of the stock material increases as the strain rate increases. In the static situation or in a slow strain rate test such as ASTM-type test, the normal ratio of yield point strength to ultimate strength would be approximately 55%. But as press speeds are increased the ratio of yield point strength to ultimate strength can increase to 80% to 85% or above. Therefore, even though the strength of the stock material has not changed, the effective shear strength of the stock material, (and yield point strength of the stock material) has effectively increased under high strain rate conditions. This again increases the loading on the press thereby causing more severe vibration.
Additionally, many dies in presses have a movable stripper that normally leads the punches, which further causes a third and fourth vibration factor. The higher above the bottom of stroke or bottom dead center (BDC) that the movable stripper plate contacts the stock material, the greater the impact effects, thus called "downward stripper impact", which further causes more press vibration. FIG. 7 depicts during press punching operation, a slide 14 and stripper plate 16 between which are connected springs 18 connected by stripper bolts 20 Bolt head 22 is in a recess 24 of the stripper plate 16. The stripper plate 16 is connected to the upper tooling 26 having multiple punches 28. Lower tooling 30, having multiple dies to punch the stock material or workpiece 32, is attached to the bolster 34 directly under the lower tooling 30. Attached to slide 14, bolster 34 and stripper plate 16 are vibration sensors 35, 36 and 37, respectively, any or all of which can be used to monitor the vibration condition of the press/die application. As the slide moves downward, the slide and stripper plate will move together as one unit until the stripper plate contacts the upper surface of the material. At that point the stripper plate does not travel any further downward but the slide and the stripper bolts continue downward compressing the spring. The higher off the bottom of stroke that contact occurs or the faster the press is running, the greater the impact force that will be created.
On the press upstroke, the slide and stripper bolts will have the same velocity. At the contact point 31 on the stripper plate 16, the mass of the stripper plate, which is at zero velocity until "upstroke impact", is accelerated to the velocity of the slide and stripper bolts instantaneously. Again an increase in press speed or increase in contact distance above BDC causes a greater impact velocity at the point at which the stripper bolts make contact with the stripper plate, thereby increasing the vibration from impact.
Another factor relative to press vibration increase is the stored energy release during the manufacture of the part. Deflections will occur on the press structure during loading of the die. As the stock material fractures through, called the snap through, the release of the stored deflection energy sends a vibration shock wave through the press structure. The released stored energy also has the ability to accelerate the slide downward, which can cause the die punches to penetrate the stock material more deeply. As the applied load increases, so does the stress and deflection levels within the press structure, therefore, causing increased energy release and increased vibration.
Another factor which affects the press structure and vibration is the use of flattening stations or stop blocks. If these devices are utilized in the die, then additional loads and impact forces are present. As press speed increases the press shutheight will naturally close in, which, if stop blocks are utilized, will cause a larger load to be applied. The press shutheight naturally closes in as press speed increases due to the inertia forces developed.
Still another factor is the thermal shutheight effect. Again, as speed is increased, there is a viscous shear of the oil within the press crankshaft bearing clearances. The heat generated from the shear of the oil is conducted through the press structure and drive connections, causing the shutheight to dimensionally close in more deeply.
Thus, the above described dynamic effects that occur during press operation increase the loading and overall vibration levels induced in the press structure, all of which increase with an increase in press speed. FIGS. 1A and 1B depict oscilloscope graphs of a press running at 100 strokes per minute and 450 strokes per minute showing press slide vertical motion 38 and its corresponding induced press vibration 40 as detected by an accelerometer 35, 36 or 37.
Vibration stress magnifications created by dynamic load increase, can cause many problems to press structures. Cracks can develop over time in the castings anywhere within the press structure or its parts if long term dynamic load increases are unknown or go ignored. Broken parts such as tie rods, crankshafts, crowns, slides and dynamic balancers have been reported, and in all instances have been able to be correlated by field service data to specific threshold vibration levels measured on the press structure during production. At certain definable vibration severity levels, stress magnification levels will be present thus creating increased maintenance severity problems for the press.
The relative life of a press is thus determinable from the accumulative effects of the vibration severity levels experienced over this period of time. A press may withstand high vibration levels without major structural damage if the duration period is relatively short. Also, a press will certainly withstand low vibration levels without structural damage no matter what the duration period. Accumulative structural damage will occur, however, when a press is run in a stressed condition as a result of medium to high vibration severity levels over a longer duration period whether run continuously or intermittently. The damage will not necessarily be evident in the early stages but will begin to appear over time.
Vibration monitoring systems of the prior art shut down the press at an individual predetermined level which when reached, would begin to cause damage to the press. The present invention measures vibration while in actual production and allows the press operator, tooling engineer, and/or production manager to know the long term reliability effects of running the press at any combination of sensed speed and load, by monitoring the actual vibration severity level of the die application, and comparing the corresponding operating vibration severity level to the produced vibration severity zone chart either manually or electronically.
The present invention advises of the predicted level of vibration severity and reliability for any application run at any speed. Previous preventive maintenance vibration monitoring only monitors no load changes to the base reference level, attained through no load reference level analysis. The previous prior art preventive maintenance vibration level measured under no load conditions do not accurately reflect actual production vibration conditions, as does the present vibration monitoring system.
Thus, for reliable long-term press production operation, a particular press must be operated within zones of safe load/speed dynamic combinations which will cause acceptable levels of press vibration severity. Each press has certain inherent characteristics which allow it to be safely operated with long term reliability within a range of production speeds and dynamic load combinations.